to John Beaumont G3NGD for information
venerable old set.
Type 391 and 394A TRF Receivers were initially used on
fishing trawlers in from 1938 omwards. They were used in
conjunction with Marconi Transmitters, Type 517 and 527.
To determine the power requirement it is important to
check the valve types as versions with 2V, 4V and 6V
filaments exist. This can be done using a Valve Data Book.
The M361 Receiver was usually supplied with 2-volt valves
as standard and these were directly heated.
The battery supply terminals are situated on the front of
the receiver and suitably labelled.
Both H.T.3 and H.T.2 terminals should be joined together
and to the positive terminal of a 120 volt H.T battery.
Terminal H.T.1 should be connected to a 72 volt H.T.battery.
The grid bias battery, which is connected externally,
supplies the negative bias required by the output valve.
The positive terminal of the grid bias battery must be
connected to the L.T. negative. For an anode voltage of
120 volts on the output Valve, then the grid bias voltage
required is approximately 3 volts.
It is very
important to check the valve types as well as the heater
voltage. If directly heated valves are used they must be
supplied with a Direct Current supply. It is very
difficult to provide this from a mains power supply.
There have been many circuits in for pure d.c.
rectification from the mains supply, but there is always
a risk of 'a.c.ripple' on the output audio. Later
receivers Marconi 394C/12 and 394C/30 used indirectly
heated valves which don't give rise to the same problems.
The receiver consists of two stages of high frequency
amplification, a detector on which reaction is available
and one output stage. The output stage may be either of
the pentode type or the power valve type, according to
the duties required of the receiver.
The aerial coupling is arranged to suit the various wave-ranges
of the receiver, and gives improved results over the
basic 394 receiver. The three tuned stages are all
carefully ganged, and a single dial only is required for
fine tuning within any given range. There are four
wavebands covered on most versions of the
receiver, but the frequency ranges can vary on different
It is important to mention however, that the fourth wave
band range is obtained, not by the use of a fourth tuning
coil, but by adding a parallel capacitor of 0.0004
microfarad capacity in parallel with the normal tuning
capacitor of approximately 0.0005 microfarad maximum
capacity. This parallel capacity must be added on all
three tuned circuits, and consequently it was necessary
to have the three 0.0004 microfarad capacitors accurate
to within 1 per cent of one another to ensure correct
obtained by feed-back through the 0.0003 microfarad
variable capacitor to a coil magnetically coupled to the
tuning coil in use in the anode of the second screened
grid valve - this gives good amplification. Volume
control is obtained by means of the grid leak bias
potentiometer, which varies
the negative voltage on the grids of the variable-mu
screened grid valves.
switch on the left-hand side of the receiver reduces the
input from strong signals if required.
A small lamp has been included in the front of the panel,
which illuminates the dial when the receiver is switched
Last updated 26th July 2003